A nuclear reactor core normally contains several hundred fuel rod bundles. Each fuel rod bundle consists of a plurality of fuel rods. Boiling water reactors normally use fuel rod bundles containing 8".times.8" fuel rods; sometimes 6".times.6", 7".times.7" or 9".times.9" fuel rods are used. Pressurized water reactors use fuel rod bundles containing 15".times.15", 16".times.16" or 17".times.17" fuel rods. One or more of these fuel rods may be substituted by inert rods or tubes having a different function than producing energy. Each fuel rod contains a large number of fuel pellets stacked on top of one another in a cladding tube, which is normally of a zirconium alloy known as "Zircaloy." In each fuel rod bundle the fuel rods are arranged between a bottom and a top plate, to which some fuel rods, so-called tie rods, are secured. In a boiling water reactor the fuel rod bundle is surrounded by a fuel channel, which is normally of "Zircaloy." Inside the channel the fuel rods are kept at the desired distance from each other in the lateral direction by spacers located at suitable distances in the vertical direction.
When the burn-up in a reactor has progressed so far that the smallest acceptable core reactivity margin has been reached, a partial recharge of fuel is carried out. By balancing, in a suitable manner, the fuel quantity to be replaced as well as the fissile enrichment of the replacement fuel, an excess reactivity is realized that permits a certain energy output until the next refuelling occasion. During the partial recharge of a boiling water reactor, it is possible to exchange, for example, one-fifth of the fuel each operating year (or any other suitable operating period), as normally from the end of the second year of operation. This means that the fuel for example remains in the core for 5 years during equilibrium conditions, but that the part of the fuel that is exchanged during the initial state is used for a shorter period of 3 to 4 years.
So far, refuelling has always been performed in such a manner that irradiated fuel rod bundles have been removed from the core and that new fuel rod bundles with unirradiated fuel have been inserted into the empty spaces formed, usually after a suitable relocation of the remaining fuel rod bundles within the core. This relocation of fuel rod bundles is done in order that the reactor may have an optimum power distribution within the core and may have optimum reactivity. The irradiated fuel rod bundles which have been removed from the reactor core then proceed to storage, awaiting ultimate reprocessing for utilizing the remaining fissile material.